<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
      div{
        width: 100px;
        height: 100px;
        border: 1px solid red;
      }
      #d1{
        /*设置绝对定位*/
        position: absolute;
        /*修改为相对定位*/
        /*position: relative;*/
        right: 0;
        bottom: 0;
      }
      #big{
        width: 200px;
        height: 200px;
        margin: 100px 0 0 100px;
        background-color: green;
        position: relative;/*做参照物*/
      }
      #big>div{
        width: 100px;
        height: 100px;
        background-color: blue;
        margin: 50px 0 0 50px;
        position: relative;/*做参照物*/
      }
      #big>div>div{
        width: 50px;
        height: 50px;
        background-color: red;
        /*绝对定位*/
        position: absolute;
        left: 0;
        top: 0;
      }
    </style>
</head>
<body>
<div id="big">
  <div>
    <div></div>
  </div>
</div>
<div id="d1">div1</div>
<div>div2</div>
<div>div3</div>
</body>
</html>